Polytetrafluoroethylene fine powder compositions are non-melt-fabricable and are commonly processed by paste extrusion wherein the powder is mixed with a lubricant and is then discharged through a paste extruder die to obtain films, tubes, tapes, protective coating on wire and the like.
To make "fine powder" PTFE, a process called "aqueous dispersion polymerization" is used. In this process sufficient dispersing agent is employed under mild agitation in order to produce small colloidal size particles dispersed in the aqueous reaction medium. In this procedure precipitation (i.e., coagulation) of the resin particles is avoided during polymerization. The dispersion may be used as such, or the dispersed particles may be coagulated in a separate step and the resulting fine powder obtained.
There is another PTFE material that is referred to by those skilled in the art as "granular" PTFE. Granular PTFE resins are employed in molding and ram extrusion processes to produce plastic articles, but cannot be paste extruded. They are prepared by "suspension polymerization" which involves polymerizing repeat units of tetrafluoroethylene (TFE) monomer in the presence of little or no dispersing agent and vigorously agitating the resulting polymer in order to produce a precipitated resin.
It has previously been known that if a small amount of repeat units of copolymerizable monomer is added to the repeat units of TFE monomer in preparing granular PTFE, the properties of the PTFE particles can be altered. U.S. Pat. No. 3,978,030 is directed to a copolymer of a dioxole and either chlorotrifluoroethylene, hexafluoropropylene, or TFE. However, a paste extrudable dioxole modified fine powder resin with superior tensile properties is not disclosed or suggested. U.S. Pat. No. 4,399,264 concerns a copolymer of perfluorodioxole (PD) and TFE. However, this reference does not disclose or suggest a PTFE core/shell fine powder composition with PD confined to a core portion and a shell portion free of PD.
It is an object of the present invention to provide a PTFE resin that is a clear, transparent material suitable for coating surfaces. It is a further object of this invention that such a material be capable of pigmenting to produce a variety of colorful coatings. These and other objects, features and advantages will become apparent in the description that appears below.